MIMO stands for “multiple input multiple output” in technical use and refers to a system or method, according to which multiple transmitting antennas and multiple receiving antennas are used for achieving an effect, for example, for transmitting information or, as in the present case, for detecting at least the location of an external object using a radar device. Transmitting antennas are designed in particular for emitting an electromagnetic signal. Receiving antennas are designed in particular for receiving an electromagnetic signal. When “antennas” are referred to here and hereafter, these are to be understood to include both transmitting antennas and receiving antennas, without differentiation.
A variety of electronic systems are used in modern vehicles, which may be used, for example, to assist a driver when driving the vehicle. For example, brake assistants may recognize preceding road users and decelerate and accelerate the vehicle accordingly, so that a predefined minimum distance to the preceding road users is always maintained. Such brake assistants may also initiate emergency braking if they recognize that the distance to the preceding road user becomes excessively small.
To be able to provide such assistance systems in a vehicle, it is necessary to detect data about the surroundings of the particular vehicle. In the above-mentioned example of a brake assistant for example, it is necessary to detect the position of a preceding road user to be able to calculate the distance of the vehicle to the preceding road user.
To detect the position of the preceding road user, for example, the azimuth angle of the preceding road user may be determined originating from the travel direction of the particular vehicle. The azimuth angle is part of an advantageous spherical coordinate system which initially relates to the radar device, but in the case of a radar device situated in a vehicle, accordingly relates to the vehicle. The azimuth angle is situated with respect to the vehicle in such a way that the azimuth angle scans a plane in parallel to the travel plane, which the vehicle is traveling, upon a variation from 0° to 360°. With the aid of the azimuth angle, for example, an object to the left of the vehicle in the forward travel direction is differentiable from an object to the right of the vehicle in the forward travel direction.
Since objects which are not functionally relevant, such as manhole covers or bridges, also reflect electromagnetic signals as radar signals, the detection of the elevation angle enables a differentiation between functionally relevant and nonrelevant objects. The elevation angle is a further part of the described spherical coordinate system and scans a plane which is perpendicular to the roadway on which the vehicle is traveling upon a variation from 0° to 360°. The last independent coordinate of the described spherical coordinate system is the distance or radius.
The azimuth angle and/or the elevation angle of an object may be detected, for example, by analyzing the electromagnetic phase offsets and/or amplitudes at the receiving antennas of a radar device of received electromagnetic signals which are reflected on the object.
US 2012/256795 A1 describes a possible antenna for such a radar device.
For a two-dimensional antenna array having phase centers xi in a first coordinate direction and yi in a second coordinate direction, which is perpendicular thereto, the following equation applies for phase φi at antenna i:
      φ    ⁢                  ⁢    i    =                    2        ⁢        π            λ        ⁢          (                        xi          *          sin          ⁢                                          ⁢          θ          *          cos          ⁢                                          ⁢          Φ                +                  yi          *          sin          ⁢                                          ⁢          Φ                    )      
In the equation, θ represents the azimuth angle and Φ represents the elevation angle.
In a general two-dimensional antenna array, azimuth angle and elevation angle have to be computed jointly. The computing effort thus increases greatly. It is therefore desirable to decouple the computation of azimuth angle and elevation angle.
Furthermore, it is desirable to manage with a preferably small number of antennas, i.e., transmitting and receiving antennas, of the antenna array of the MIMO radar device. The application of the conventional MIMO principle combines reception signals of multiple switching states and thus enables the formation of virtual arrays having an enlarged aperture of a high number of virtual antennas.
The formation of virtual arrays av(θ) is carried out by convolution of the receiving antenna array, which is made up of the receiving antennas, with the transmitting antenna array, which is made up of the transmitting antennas, i.e., by forming all possible products of one-way antenna diagrams atx(θ) of the transmitting antennas with one-way antenna diagrams arx(θ) of the receiving antennas:av(θ)=atx(θ)arx(θ),(X) symbolizing the Kronecker product, i.e., forming all possible products of the elements of vectors atx(θ) and arx(θ).
Objects moving in relation to the radar device result in a phase offset between transmitting states in time multiplex MIMO, because of which compensation methods are applied for a compensation of the phase offset.